Honors Chemistry – Unit 5 Chemical Equations Chapter 10

What is a chemical rxn? A process where one or more substances changes to one or more other substances. Exchange of electrons!!! This is NOT a nuclear rxn! All atoms remain the same – they just rearrange

Parts of a chemical reaction Reactants the original substances Products the resulting substances

ASK YOURSELF: Which are the products and which are the reactants in these equations? Na + + Cl -  NaCl 2H 2 O  2H 2 + O 2

Quick Demo Magnesium

A word equation Magnesium metal reacts with oxygen gas to form white magnesium oxide solid. We must learn how to translate these!

Ways to Represent Reactions Chemical equation: symbols that describe a chemical reaction and show what atoms (and how many, relatively speaking) are involved. Word equation: the names of each product/reactant are written out. Doesn’t show how many of each

Write out the word equation Magnesium metal reacts with oxygen gas to form magnesium oxide solid. Mg + O  MgO

Remember BrINClOFH Br 2 – a liquid I 2 - a solid N 2 - a gas Cl 2 - a gas O 2 - a gas F 2 - a gas H 2 - a gas

Check for BrINClOFH Any time you see 1 of the 7 diatomic elements alone, write them with a 2 subscript Mg + O 2  MgO

Balance the charges of ionic comp In our reaction we have already done this, but make sure to use proper formulas (for molecular comp too!): MgO = Mg 2+ O 2- Mg + O 2  MgO

Balance number of atoms/ions on each side In ordinary chemical changes, atoms just rearrange This is the LAW OF CONSERVATION OF MATTER We do this with coefficients 2Mg + O 2  2MgO

Example If you take Ca(NO 3 ) 2 and change it to 2Ca(NO 3 ) 2, how many of each element do you have?

Put in physical states Symbols used in chemical equations: (s) solid (l) liquid (g) gas (aq) aqueous solution (dissolved in H 2 O)

Put in physical states I will give you physical states in word problems: Magnesium metal reacts with oxygen gas to form magnesium oxide solid 2Mg(s) + O 2 (g)  2MgO(s)

Balancing equations Tips __Fe 2 O 3 + __H 2 SO 4  __Fe 2 (SO 4 ) 3 + __H 2 O If possible treat polyatomic ions as 1 unit Balance hydrogen 2 nd last Balance oxygen last If the last element does not balance double everything

Word Equation example Hydrogen gas and Oxygen gas combine to form liquid water. 2H 2 (g) + O 2 (g)  2H 2 O(l)

Example: Sodium solid reacts with chlorine gas to form sodium chloride solid. 2Na(s) + Cl 2 (g)  2NaCl(s)

Try this one: Dicarbon dihydride reacts with oxygen gas to produce carbon dioxide gas and water

Evidence of rxns All rxns involve chemical change Look for evidence of those changes: Light, heat, gas, color change, precipitate

Other Symbols  precipitate forms  gas forms  “produces” or “yields”  products can reform in to reactants – end result is a mix of the two. Δ – means that heat was added

Chemistry – Chemical Equations Part II

Experiment 17 Please read over steps 1-8 carefully Get the proper safety gear, and the proper equipment Do steps 1-8, then set the solid aside to dry (step 9) Return to your seat – while the solid dries, you can work on A7 At the end of class – record the mass of your beaker and solid.

Balanced equations show RATIOS The coefficients of a balanced equation reveal the RATIO of each reactant to each product: 2H 2 + O 2  2H 2 O means there is a 2:1:2 ratio of H 2 to O 2 to H 2 O. This is the same as a 4:2:4 ratio, etc. etc.

The activity series Writing an equation doesn’t mean it will actually happen How do we know it will happen? Single replacement reactions are predicted with the activity series This activity series has been created through experiment.

The activity series Things on the list will displace anything below but not above other things on the list There are two lists – one for elements that become positive ions, and one for elements that become negative ions.

ElementReactivityElementReactivity Li Rb K Ba Ca Na React with cold H 2 O and acids, replacing hydrogen F 2 Cl 2 Br 2 I 2 Listed from most reactive to least reactive Mg Al Mn Zn Cr Fe React with acids or steam but usually not liquid water, to replace hydrogen Ni Sn Pb All react with acids but not water, to replace hydrogen H 2 Cu Hg All react with oxygen to form oxides Ag Pt Au Mostly unreactive

? When a rxn releases energy, where does it come from? You have to put energy in to break bonds If bonding is stronger in the products they will be more stable More stable = lower energy Lower energy means energy must be RELEASED Is this an example of an ENDOthermic or EXOthermic reaction?

If bonding is weaker in the products, they will be less stable, have more energy, and will therefore absorb it. Is this an endo or exothermic rxn?